1. Physics 1230: Light and Color Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Phone:
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
Class # 24
Chapter #13
Reminder: HW #7 is due on Thursday
Today: Finish Ch.13 material & Review
Thursday: Exam #3

2. Physics 1230: Light and Color Chapter 13
Scattering and Polarization
Light waves can scatter off water droplets, dust particles and other molecules in air
Some wavelengths in white light scatter more than others (Rayleigh scattering)
This is why the sky is blue
The polarization of light refers to the plane in which the waveform and electric force field lies
Most light is unpolarized
The waveforms and electric force field on a given ray keep jumping around in orientation while always perpendicur to the ray
Scattered or reflected light is polarized in particular planes
Polaroid sunglasses can block out polarized light from sky and ground
3D movies & other applications of polarizers;

3. How do 3D movies use polaroid filters (polarizers)?

4. What can a light wave do when it encounters matter?
Be TRANSMITTED
laser aimed at water or glass
Be REFLECTED
specular reflection of light by a mirror
diffuse reflection of the light in this room off all the other students
reflection is re-radiation of light by the electrons in the reflecting material
Be ABSORBED
Cyan light shining on a red apple is absorbed by electrons in the apple
Something new!!
A light wave shining on molecules in the air or plastic or other “transparent” materials can be
SCATTERED
Light ray moves over to the side in all directions rather than forward, backward or being absorbed.
Intensity of the scattered light can depend on wavelength

5. What is Rayleigh scattering? (or why is the sky blue)
The shorter the wavelength, the more light is scattered
blue is scattered more than red.
this is why the sky is blue and sunsets are red. (Fig. 13.1)
Think of white light from sun as a mixture of R, G and B
Blue is scattered the most so sky looks blue when we look away from the sun
For same reason sun looks yellow (red + green)
More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks red

6. Rayleigh scattering But why blue and not violet?
Intensity inversely proportional to λ4
Light waves with longer wavelength scatter more
But why blue and not violet?

7. Why blue and not violet? Spectral response of cones
in typical human eye
relative response
Our eye sensitivity to the blue color is much stronger than that to violet!
We perceive the deep waters being of blue color for the same reason!

8. Color of Day-time sun vs. Sunset
Rays from Sun (not scattered)
White or yellow
Earth
Atmosphere

9. Red Sun/sky during the Sunset

10. Why the sky is blue?
A. Because of scattering of short-wavelength blue light coming from the Sun;
B. Because of light absorption similar to that in color filters;
C. A & B
D. Because of dispersion and total internal reflection;
E. None of the above;

11. What color of the sky one can see from Jupiter?
D. Black
A. Red;
B. White;
C. Blue;
E. Yellow;

12. Without Rayleigh scattering the Sky would be
A. Red;
B. Green;
C. Blue
D. Black;
E. White;

13. Light scattering in milk and smoke?
Larger particles scatter red as well as blue and hence look white.
Dust or smoke
Clouds;
Milk;
Colloidal suspension
There are many
other types of
scattering…

14. Why the sunset is red?
A. Because of light absorption similar to that in color filters;
B. Because of scattering of blue and green light and only red light coming to our eyes;
C. Because of its polarized nature;
D. Because of chromatic & spherical aberrations;
E. None of the above;

15. Polarized Light due to reflection
Secularly reflected light blocked using a polarizer (Polaroid film)

16. Amplitude (maximum height)
Review: What is the wave of a mono-chromatic (single wavelength) wave along a ray?
Speed of light in empty space is c = 186,000 miles/sec = 3 x 108 meters/sec
Amplitude (maximum height)
The intensity of the light is proportional to the amplitude squared. Large amplitude means bright light. Low amplitude means dim light
Ray
Wavelength
Waveform

17. Review: What is light "made of?"
Light consists of electric (and magnetic) fields moving through space at the speed of light
The waveform can be replaced by a set of arrows perpendicular to the ray whose tips lie along the waveform

18. Crossed polarizers: block propagation of electromagnetic waves
Waves on a rope
Think about the waves on a rope:

19. For light: Polarizers (Polaroid films)
Same story:

20. What is polarized light?
Light is polarized if the waveform and electric force field arrows remains in the same plane
The (green) electric force arrows must always be perpendicular to the ray
This is a light ray traveling in the z-direction and polarized in the y-direction
Here is a light ray traveling in the same direction but polarized in the x-direction
We will visualize the polarization in the x-y plane, looking at rays head-on
The green force arrows point up and down or left and right, stacked up behind one-another.
Here is the convention for visualizing vertical and horizontal polarization
Looking at ray "head-on" see green arrows up & down y z x y z x y x

21. In which direction is this light ray polarized?
A. Vertical;
B. Horizontal;
C. Along the light propagation direction (yellow arrow);
D. None of the above;

22. What is unpolarized light?
For unpolarized light the plane of polarization keeps jumping around
But the electric field arrows remain perpendicular to the ray (direction of travel of the wave)
We visualize this in the x-y plane (looking into the ray) as shown at right
The many crossed double sided arrows are the symbol for unpolarized light
See Physics 2000
electric force arrows jump around while remaining perpen-dicular to the ray x y z
wave travels in z-direction y x

23. When unpolarized light reflects off a horizontal surface (such as water or beach) near a special angle, the reflected light is polarized in the horizontal direction
The special angle of incidence is where the refracted ray and reflected ray are perpendicular to each other
This is called Brewster's angle
To understand, imagine the electric force arrows of the incident unpolarized light to be decomposed into two perpendicular polariza-tions
the first polarization is horizontal (force arrows are parallel to the flat reflecting horizontal surface and perpendicular to the ray)
in the 2nd (Fig. 13.5), the arrows are perpendicular to both the ray and the horizontal force arrows
The second polarization cannot be sustained in the reflected ray because the force arrows would be parallel to that ray (impossible for a light ray)
Hence, only the horizontal polarization survives in the reflected ray

24. Polaroid sunglasses (and camera filters) take advantage of this effect to block bright reflected light from the snow, beach, road, water, sky, etc.
When you wear polaroid sunglasses they block light polarized in the plane of your two eyes
When your head is vertical this is the horizontal plane
Since reflected light from the ground or scattered light from the sun are mostly polarized in the horizontal plane such light is mostly blocked
Demo with polaroid filters
The first filter blocks one polarization and lets through the other (perpendicular) one
The second blocks more and more of the remaining polarized light as its axis is rotated so as to permit only the polarization blocked by the first
When the axes of the two polaroid filters at at right angles to each other no light comes through
Physics 2000 polarization applets
The polaroid filter works because it has long molecules lined up parallel to each other like hairs
Components of light with polarization parallel to the molecules are absorbed and components of light polarized perpendicular to the molecules are absorbed and don't go through
Other demos with crossed polarizers
Third polarizer at angle in between lets light through again
Stressed plastic between polarizers shows colors which reveal the stress patterns

25. Some material from Chapter #8

28. How do 3D movies use polaroid filters?

29. A different approach:
Use color filters to make the left and right eyes perceiving slightly differen images

30. Clicker Question: In 3D movies:
Left eye sees the same as the right eye;
Left eye sees the inversed image compared to that seen by the right eye;
The images seen by the two eyes are slightly different and taken from different angles.